To query the customers table by ID and only get one result back, you can use customerTable.LookupAsync(customerId) - this will throw an exception if it fails to find a record with the specified ID so its more like Single rather than SingleByDefault.
You mentioned that you want SingleOrDefault, but the code in your constructor assumes that you have a valid reference, so this makes me think that LookupAsync is exactly what you want (better to get an exception that says "no customer with ID: 999 found" than simply a NullReferenceException).
One more tip: don't refresh the entire Azure portal when you just want to see if your data has been saved! There is a refresh button on the bottom of the page when you have the browse tab open. Its much faster
Hope this helps,